It is well known that adapting transmission parameters to the current channel conditions improves the performance of a wireless communication system. Parameters that can be adapted are for example the power allocation, both in the frequency domain and on different antennas in a multi antenna system, as well as modulation, coding, etc.
With channel state information (CSI) at the transmitter it is possible to allocate the available power to the parts of the frequency spectrum that have good channel conditions, i.e. not waste the power on frequencies that are in deep fade for the moment. On the parts of the spectrum that have good channel conditions it is also advantageous to use higher order modulation and lower coding rate.
In future mobile systems larger bandwidths will be used and therefore broadband antennas. The gain of these broadband antennas are however not equal over the whole frequency range. In a handheld unit the gain on different frequencies will also change when the antenna interacts with the user. In a system where the CSI is reported by a receiving unit to a transmitting unit, the antenna gain will be incorporated in the reported CSI.
In systems with multiple transmit antennas it is also known that the capacity of the system is improved when the transmission parameters are adapted to the current channel conditions for each antenna. Once again not to waste power on an antenna that is in a deep fade or for some other reason have poor channel conditions.
An optimal power allocation can only be achieved when the transmitter has full CSI. Without CSI at the transmitter it is not possible to adapt to the current channel conditions at the transmitter and the best strategy is to transmit equal power, same modulation format and same coding rate over the whole frequency band and at all antennas, as illustrated in FIG. 5.